Background: Air pollution has been associated with hypertension and preterm birth. We examined if prenatal exposure to air pollutants was associated with gestational hypertension and if its association with preterm birth was modified by maternal hypertension.
Methods: Data were from birth certificates and hospital discharge records of 252,205 women in San Joaquin Valley of California from 2000-2006. Air quality data were assigned from 24-hour averages of nitrogen dioxide (NO2), particulate matter <10μm (PM10) and <2.5μm (PM2.5), and carbon monoxide (CO) for different averaging periods over pregnancy. We estimated odds of preterm birth and multiplicative interaction between each pollutant and hypertensive disorder.
Results: Among normotensive women, odds of preterm birth were slightly higher for higher exposure to all pollutants over the entire pregnancy. Patterns were similar among women with a hypertensive disorder. Among 32-36 week births there was effect modification for exposure to NO2 and CO during the first trimester with higher odds among hypertensive women, and PM2.5 and CO during the last six weeks with higher odds among normotensive women. For 28-31 week births, there was effect modification by hypertensive status for PM10 exposure for entire pregnancy, first, and second trimester with hypertensive women consistently having lower odds of preterm birth than normotensive.
Conclusion: There was some evidence of effect modification in the direction counter to our hypothesis for exposure to PM10 and early preterm birth, and CO and PM2.5 at the end of pregnancy, but overall, hypertension did not modify the relationship between pollution and preterm birth.